This invention relates to an induction motor drive apparatus, and more particularly to an induction motor drive apparatus which is capable of reducing noise during operation under a light load.
It is well known in the art that variable frequency (VF) control or variable voltage-variable frequency (VVVF) control can be applied to drive and control induction motors by converting a direct current into alternating current through an inverter circuit and then utilizing the alternating current to operate the motor. With variable frequency VF control the primary frequency of the inverter circuit, namely the output of the inverter circuit, is varied in accordance with a command speed. With variable voltage-variable frequency VVVF control, on the other hand, the amplitude of the primary voltage is also varied in proportion to the change in the primary frequency, so that the output torque is held constant.
In accordance with these conventional control systems the speed of an induction motor is controlled by adjusting the amplitude and frequency of the primary voltage. However, since such systems achieve control on the basis of mean values, it has not been possible to achieve instantaneous control with a high degree of response. Pulse width control systems have recently been employed in an effort to improve upon the systems described. This has led to the development and application of a so-called "vector control system" which utilizes pulse width control to control an instantaneous value of an induction motor stator current so as to enable the generation of a torque which is truly equivalent to that produced by a shunt DC machine. See, for example, "Induction Motor Vector Control" in the magazine Yasukawa Denki, No. 4, 1974, pages 597 through 599 (published by Yasukawa Denki). However, even these new systems employ a pulse control method which is similar to the conventional slip control method, with the result that a considerable level of excitation noise is generated during induction motor operation. When the induction motor is operating under a heavy load the excitation noise is not particularly noticeable since it is drowned out by the cutting noise produced by a machine tool or the like. The excitation noise can, however, become quite annoying to personnel in the immediate vicinity if the induction motor is idling or running under a light load.